This application is based upon and claims priority from Japanese Patent Application No. 2001-84475, filed in Mar. 23, 2001, the content being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor device using a low dielectric constant film and a silicon-based composition used for the same, a low dielectric constant film, and a method for producing the low dielectric constant film.
2. Description of the Related Arts
It is important for integration and miniaturization of a semiconductor to reduce the parasitic capacitance that is generated between wirings in multilayer wiring processes of a semiconductor integrated circuit and to reduce the delay of the signal propagation speed (i.e., the wiring delay).
Although the reduction of the signal propagation speed by the parasitic capacitance of an insulating film has been recognized so far, the influence of the wiring delay on the entire device has not been so remarkable in the semiconductor devices of generations in which the wiring gap is larger than 1 xcexcm.
In the case the wiring gap is 1 xcexcm or less, however, the influence on the device speed becomes large. In particular, when a circuit is formed with the wiring gap being 0.5 xcexcm or less as will be expected in near future, the parasitic capacitance between wirings will be affecting the device speed more. Accordingly this will be a big obstacle for the integration and miniaturization of a semiconductor.
In other words, while the reduction of the signal propagation speed depends largely on the wiring resistance and parasitic capacitance between wirings in the multilayer wiring of a semiconductor integrated circuit, higher integration of a device makes the width of a wiring and wiring gap narrower, resulting in increase in the wiring resistance and parasitic capacitance between wirings.
The capacitance of an insulating film can be reduced by making the wiring thickness thinner to reduce the cross-sectional area. However, making the wiring thinner results in larger wiring resistance, and therefore, a higher signal propagation speed cannot be achieved.
Accordingly, it is indispensable for achieving a higher signal propagation speed to make the resistance of a wiring and the dielectric constant of an insulating film lower, and it is expected that they will play very important roles in deciding properties of a device in future.
Wiring delay (T) is affected by wiring resistance (R) and capacitance (C) between wirings as shown in Eq. 1.
Txe2x88x9dCRxe2x80x83xe2x80x83(1) 
In eq. 1, the relation between e (dielectric constant) and C is expressed by Eq. 1xe2x80x2.
C=∈0∈rxc2x7S/dxe2x80x83xe2x80x83(1xe2x80x2) 
wherein S is an electrode area; co is a dielectric constant of the vacuum; ∈r is a relative dielectric constant of an insulating film; and d is a wiring gap.
Therefore, the wiring delay is effectively diminished by making the dielectric constant of the insulating film lower.
Inorganic films such as silicon dioxide (SiO2), silicon nitride (SiN), and phosphate silicate glass (PSG) and organic polymers such as polyimide have been used as insulating materials so far.
However, the dielectric constant of CVD-SiO2 films, which are most frequently used for semiconductor devices, is about 4 or so. Although the dielectric constant of an SiOF film, which is now being calling attentions as a low dielectric constant CVD film, is about 3.3-3.5, it is hygroscopic, so that it has a problem in that the dielectric constant is increased by absorbing water.
In addition, as a low dielectric constant film, a porous film made of a siloxane resin having SiH bonds is known. However, when a semiconductor device is subjected to washing with an alkaline solution, there is a problem in that highly hygroscopic SiOH groups are formed as a result of hydrolysis, resulting in an increased dielectric constant, together with the problem of mechanical damages on the semiconductor part such as cracks caused by the washing. To solve these problems, a protecting film such as a SiO2 film has been formed conventionally. However, this will make relatively smaller the rate of the low dielectric constant film in a semiconductor device, and accordingly, the effective dielectric constant will be increased when multilayer wirings are formed.
To compare, organic polymer films can be used to have a lower dielectric constant. However, the glass transition temperatures are as low as 200-350xc2x0 C. and the coefficients of thermal expansion are large, so that the damages to the wiring are problematic.
It is, therefore, the object of the present invention to solve the above-mentioned several problems in order to form an excellent film, as well as to provide an insulating film having a lower dielectric constant than those of conventional insulating films, and to provide fast and reliable semiconductor devices.
In addition, the present invention improves, in many cases, chemical resistance, especially against alkali solutions of a silica-based film, and it can solve the problem of highly hygroscopic characteristic presented in conventional porous films made of a siloxane resin.
According to one aspect of the present invention there is provided a composition comprising a siloxane resin, a silicon compound substantially consisting of silicon, carbon and hydrogen, wherein the number ratio of carbon to silicon atoms forming an xe2x80x94Xxe2x80x94 bond (wherein X is (C)m (where m is an integer in the range of from 1 to 3), or a substituted or unsubstituted aromatic group with carbon atoms of not more than 9) in the main chain of one molecule is from 2:1 to 12:1, as well as a solvent. Hereupon, it is to be noted that xe2x80x98Cxe2x80x99 of (C)m means a carbon atom.
As other aspects of the present invention, there are provided a low dielectric constant film obtained by subjecting the composition to a heat treatment, a semiconductor device having the low dielectric constant film as an interlayer insulating film, and a method for producing the low dielectric constant film.
It was found that a film obtained by adding a silicon compound having a silicon-carbon bond(s) in the skeletal chain (main chain) to a siloxane resin is given a nature of repelling chemicals such as an alkali.
It was also found that, when the compound was added to a siloxane resin, the compound was homogeneously dispersed into the siloxane resin because of its high compatibility, and that the resistance against acidic and alkaline solutions was improved and was lasting even if the compound was added to the siloxane resin at a weight ratio of 0.1 part by weight based on 100 parts by weight of the siloxane resin.
It was also found that a silicon compound having a silicon-carbon bond(s) in its skeleton had a high moisture resistance, and therefore, the composition according to the present invention was effective even in forming porous films that would have a problem of low moisture resistance, unless it were used.
It was also found that the combination of a siloxane resin and such a silicon compound can prevent damages (mechanical damages such as cracks) caused on a siloxane-based low dielectric constant film in an alkaline solution, which has been a problem for low dielectric constant films having a SiH bond(s), and that the increase in the dielectric constant caused by the hygroscopic behavior can be solved in many cases, which has been another problem for low dielectric constant porous films.
Such a silicon compound can be identified as a silicon compound having an xe2x80x94Xxe2x80x94 bond (wherein X is (C)m (where m=1 to 3) or a substituted or unsubstituted aromatic group with 9 or less carbon atoms) in its main chain.
By using s silicon compound having the above-mentioned xe2x80x94Xxe2x80x94 bond (wherein X is (C)m (where m=1 to 3) or a substituted or unsubstituted aromatic group with 9 or less carbons) in its main chain, together with a siloxane resin, forming a coating film containing both, and heating the resultant film, it is possible to produce a low dielectric constant film with minimized hydrolysis and/or damages by the chemical treatment, while the elevation of the dielectric constant in a process for producing a semiconductor is suppressed.
It is preferable that the above-mentioned silicon compound has a structure represented by formula 2 below: 
(wherein R4 and R5 are each, same or different, H or an aliphatic hydrocarbon group with 1 to 3 carbon atoms or a substituted or unsubstituted aromatic hydrocarbon group with 6 to 9 carbon atoms; R6 is an aliphatic hydrocarbon group with 1 to 3 carbon atoms or a substituted or unsubstituted phenylene group; and p is an integer of 20 to 1,000).
Limitations on R4, R5 and degree of polymerization are necessary to keep the viscosity of the composition before forming a film in an appropriate range, while the limitation concerning R6 is important to secure the heat resistance of the formed low dielectric constant film.
It is preferable that the above-mentioned siloxane resin has a structure represented by formula 3 below: 
(wherein R1, R2, and R3 are each, same or different, hydrogen, fluorine, a methyl group or an xe2x80x94Oxe2x80x94 bond; and n is an integer of 5 to 1,000).
It is not much preferable that R1, R2, or R3 is an aliphatic hydrocarbon group other than hydrogen, fluoride, or a methyl group since the elimination of hydrogen is enhanced, resulting in acceleration of the formation of cross linkage. It has been found that, although the xe2x80x94Oxe2x80x94 bond also promotes crosslinking reactions, they do not cause practical problems.
The xe2x80x94Xxe2x80x94 bond used herein (wherein X is (C)m (where m is 1 to 3) or a substituted or unsubstituted aromatic group with 9 or less carbon atoms), taking the case of m=3 for example, means a state in which three carbon atoms are serially linked as Cxe2x80x94Cxe2x80x94C, wherein each carbon can have a substituent group(s) other than hydrogen. A simple example of the case of m=3 is propylene group. The m is limited to the range of from 1 to 3 because the decrease in heat resistance may be observed when m is 4 or larger. Phenylene group is an example of the substituted or unsubstituted aromatic group with 9 or less carbon atoms.
Tests revealed that a silicon compound substantially consisting of silicon, carbon and hydrogen, wherein the number ratio of carbon to silicon atoms forming an xe2x80x94Xxe2x80x94 bond (wherein X is (C)m (where m is an integer in the range of from 1 to 3), or a substituted or unsubstituted aromatic group with 9 or less carbon atoms) in the main chain of one molecule is from 2:1 to 12:1, is preferable, since it gives a low dielectric constant film having a good balance of low dielectric constant, chemical resistance against acids and alkalis, and moisture resistance.
From the viewpoint of the production method, the above-mentioned siloxane resin is preferably a siloxane resin obtained by subjecting to a heat treatment a mixture containing a tetraalkoxysilane (a) and an alkyltrialkoxysilane and/or trialkoxysilane (b) at a molar ratio (a:b) of 0:1 to 1:0.
In order to obtain a siloxane resin having consistent qualities, it is preferable to produce the resin by releasing, during the above-mentioned heat treatment, from 100 to 400 moles of alcohols out of 100 moles of (a+b), the total of the tetraalkoxysilane (a) and the alkyltrialkoxysilane and/or trialkoxysilane (b).
It has been also found that it is preferable that the carbon concentration in the siloxane resin is in the range of from 1 to 80 atom % per total atoms in the siloxane resin to achieve a consistent low dielectric constant. It is conjectured that hydrolysis is suppressed upon formation of the low dielectric constant film.
It has been also found that it is similarly preferable that the concentration of hydrogen atoms directly bonded to silicon in the siloxane resin is in the range of from 1 to 25 atom % per total atoms in the siloxane resin.
A case in which both conditions are satisfied is also one of the preferable embodiments.
It is preferable that the weight ratio of the above-mentioned silicon compound to the above-mentioned siloxane resin is in the range of from 0.001 to 2, that is, 0.1 to 200 parts by weight of the former based on 100 part by weight of the latter.
It is preferable that the composition according to the present invention also contains a substance selected from the group consisting of a novolak resin, an epoxy resin, an acrylic resin, a polyester, polypropylene, a phenol compound, an imidazole compound, and an adamantane compound.
It has been found that an acrylic resin is especially preferable among the above-mentioned substances.
There are various kinds of acrylic resins with respect to the degree of polymerization, the level of copolymerization, and so on. An appropriate one can be selected among them by a trial and error method.
These substances are added to form pores by the vaporization and dissipation thereof while a low dielectric constant film is being formed. That is, they are dissipating agents (pore-forming substances).
Although a low dielectric constant film having a further lower dielectric constant can be produced by the formation of pores, mechanical properties such as strength are deteriorated at the same time. Therefore, it is desirable that pores having appropriate sizes are distributed in an appropriate manner.
It is preferable to add from 5 to 200 parts by weight of a dissipating agent to 100 parts by weight of a siloxane resin. This is because the dielectric constant is not reduced enough when it is less than 5 parts by weight, and the strength of the film is reduced when it is more than 200 parts by weight.
It has been also found that a substance having a thermal weight loss coefficient in a specific range is appropriate as the above-mentioned substance. Namely, a substance is desirable which loses its weight by 5% by weight or more at 150xc2x0 C. and loses its weight by 90% by weight or more at 400xc2x0 C. when it is heated up from usual temperature at a rate of 10xc2x0 C./min. This is probably because formation of the above-mentioned pores is appropriately controlled.
It is desirable that the porosity of the above-mentioned pores is from 10% to 70% by volume per the total volume of the low dielectric constant film. When the porosity is less than 10% by volume, its porosity-generating effect is small. When the porosity exceeds 70% by volume, mechanical properties are deteriorated. A desirable average pore size (diameter) is in the range of from 50 to 200 nm.
The above-mentioned examinations have revealed that a low dielectric constant film having an SiO4 bond(s), a Cxe2x80x94SiO3xe2x80x94 bond(s), as well as an xe2x80x94Xxe2x80x94 bond(s) (wherein X is (C)m (where m is an integer in the range of from 1 to 3), or a substituted or unsubstituted aromatic group with 9 or less carbon atoms), and having a porosity of from 10% to 70% by volume and a relative dielectric constant of from 1.4 to 2.5 achieves a low dielectric constant, suppresses the hydrolysis, and has good mechanical properties. A low dielectric constant film having such properties can be achieved by combining a film with a composition comprising a siloxane resin and a silicon compound having a silicon-carbon bond(s) in its skeletal chain, with pores.
As a representative mechanical strength, it is desirable that the tensile strength at break is from 30 to 80 MPa as measured by the Stud Pull method. A tensile strength at break of smaller than 30 MPa is not mechanically sufficient. When it exceeds 80 MPa, it is not inconvenient from a mechanical viewpoint, but the dielectric constant will be deteriorated in many cases.
The present invention has been achieved as a result of investigations of a system having an SiO4 bond(s), a Cxe2x80x94SiO3xe2x80x94 bond(s), as well as an xe2x80x94Xxe2x80x94 bond(s) (wherein X is (C)m (where m is an integer in the range of from 1 to 3), or a substituted or unsubstituted aromatic group with 9 or less carbon atoms). However, when the achieved results are observed only from the viewpoint of physical properties, it can be considered that a low dielectric constant film having a porosity of from 10% to 70% by volume, a relative dielectric constant of from 1.4 to 2.5, and a tensile strength at break of from 30 to 80 MPa as measure by the Stud Pull method, and consisting substantially of silicon, carbon, hydrogen and oxygen, provides the above-mentioned various effects.